1. The Field of The Invention
The present invention concerns a device for measuring the velocity of ultrasonic vibrations in a moving web such as a sheet of paper.
2. State of the Art
In the manufacture of paper it is often important to know the strength of the paper. Strength of paper can be tested in various ways including burst strength, tensile strength and pierce strength. Normally, these strength parameters are determined by various destructive tests in a laboratory. However, on-line determination of paper strength during the manufacture of the paper can aid in producing paper which can meet specified strength requirements.
U.S. Pat. No. 4,291,577, assigned to the Institute of Paper Chemistry, teaches a system for measuring the strength of paper as it is being produced. The patent provides for measurement of the velocity of ultrasound waves through the moving paper sheet. Based upon the velocity the strength of the paper can be determined.
The patent teaches a device having two wheels which are spaced apart from one another and which roll along the moving paper web. The first wheel contains a transducer in the form of a rectangular button mounted on the periphery of the wheel so that as the wheel rotates the button periodically contacts the paper. Each revolution of the wheel, when the transducer contacts the paper, it receives an electrical signal from a signal generator and imparts a mechnical signal to the paper. The second wheel contains a receiving transducer substantially the same as the transmitting transducer, which also is mounted on the periphery of the wheel and occupies a small percentage of the total circumference of the wheel. The receiving transducer contacts the paper once each revolution of the wheel and receives the signal from the transmitter by picking up the ultrasound signal from the paper and converting it to an electrical signal. The system also includes a position detector to monitor the rotational position of the first wheel and to trigger the firing of the ultrasound pulse by the transmitter when the wheel is in a predetermined position. The rotation of the receiving wheel is coordinated with the transmitting wheel so that the receiving transducer is in contact with the paper at the appropriate time to receive the transmitted signal. The signal from the receiving transducer is transmitted to a metering and recording apparatus which measures the velocity of the ultrasonic waves.
The system described in the patent has a number of disadvantages. Since the transmitting transducer and the receiving transducer each contact the paper during only a small percentage of the total rotation of the wheels, the rotation of the wheels must by carefully synchronized and the transmission of the pulse must be carefully timed so that the pulse is received by the transmitter when it is in contact with the paper. Furthermore, since the transmitter and the receiver are only in contact with the paper for a small percentage of the total rotation of the wheels, a substantial portion of the paper is not subject to velocity measurement.